2004 XJ8, are the plastic intake manifold prone to crack and leak. I know the plastic transmission pans are, On a cold start the STFT goes to 25 both banks. At operating temperature fuel trims are fine. So I suspect a vacuum leak when cold and it closes up when heat expands something. What could it be? The plastic intake manifold is my thought. Smoked it, tried propane, brake cleaner, nothing. Any thoughts?

 

The eight gaskets inside the intake manifold, was where my vacuum leak, was finally found.

 

It's not a known problem but I am sure it could split.

The transmission pan is usually wet with fluid because of the leaking sealing sleeve and NOT a cracked pan.
Again it can happen but look at that sealing sleeve first.

I would check the P/B booster vacuum connection on the manifold and inspect the plastic air ducts from the air box to the intake for cracks.
But without monitoring short and long term fuel trims you are going to be lost. I had major vacuum leaks on my old STR and spraying stuff never worked UNTIL I was monitoring fuel trims.
Then the STFT and the LTFT went crazy when I hit the right spot. These cars can compensate so well that the engine just keeps running fine but the fueling is all over the place.
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Hi Tom,

We have had a report from one of our members of a crack in the intake manifold. I seem to recall that it was low, near one of the mounting screws, and it was discovered by smoke testing. But you would expect a cracked manifold affect fuel trims at all operating temperatures, unless the plastic was expanding when hot and closing up the crack.

I would be more inclined to think of systems that operate only at startup, such as the Secondary Air Injection control valve, or that are not supposed to be open at startup but might be, such as the Evaporative Emissions carbon canister purge valve. Another thing to ponder is that at startup the oxygen sensors are ignored briefly while their heaters get them up to temp. If your scan tool can read the open-/closed-loop PID, watch to see if the fuel trims come down as soon as the system goes into closed-loop fuel metering.

You don't mention any diagnostic trouble codes. I would think that with fuel trims at +25 at startup you must at least have P0171 or P0174 or both, and possibly other codes that might be helpful in the diagnosis. Can you tell us all the codes you have?

I just went through the process of diagnosing high positive fuel trims at idle. Here's how I approached it and what I discovered:

High fuel trims with P0171 and P0174

Cheers,

Don

 

Good suggestions, I will investigate those. I use to get 171 & 174, I found and fixed some leaks. The engine goes closed loop in less than a minute and STFT come down quickly. I wondered abut the EVAP system. I don't even know what or were the secondary air injection control valve is. I will review the manual and investigate that also. Thanks for the suggestions.

 

where were you able to find the gaskets? I have the same issue and my shop cannot find them .

 

Found mine on ebay three years ago, found these today.


Intake Gaskets for Jaguar XJ8

Jaguar xj8 4.2L Upper Intake Manifold Gasket (set of 8 seals) Intake Gasket

Fits Jaguar XJ8 4.2 Liter and S-Type 4.2 Liter!
$64.95
Brand: Jaguar
FAST 'N FREE
Material: Rubber
Watch

 

Thanks for the response Wingrider. I saw those and sent to my Mechanic but he did not think those were for the 2007 Model. He claims mine should be a Orange/Brown Color and do not have the black inserts on the sides.
I plan to go there tomorrow to look at what he is seeing.
Just curious, do you remember if your original was the green color?

 

The original ones in mine were a grey color, the ones used to replace were green like these.
Just do the job yourself, & save some cash.

 

Wingrider speaks truth! While its not always possible to do the work, the return for your labors and research go beyond the cost, which alone is a great motivator. The job gets done right, the money saved allows you to build a useful set of tools. You also learn about the engineering and design incorporated in these cars.

I generally perform all work on my families vehicles, in my driveway, except mounting tires, items needing a press or a lift, which aren't occurrences. This forum and YouTube have forever changed the fear factor of doing the work, shorten the work times, as well as saving you from repeating history by illustrating what NOT to do. Even when not video or pictorial is available the information is there for just about any job you might need to perform.

FYI the intake manifold on AJ V8 engines is reinforced Polyamide composite plastic. Doing the gaskets seems like your best move at present, assuming all soft lines are leak-free.

 

For those that are watching this thread, they did change the design for the later models around 2006. My 2007 is an red/Orange gasket and does not have the black tabs on the side. Thankfully, it is the same design as used on some of the Land Rover models of the same year.
The part number for any one that needs them is LR006678 and they are widely available.
I am envious of those that do their own work but I do not currently have the space, tools, testing equipment, or skills. To Blairwares point, if I plan on keeping the car, I probably need to start learning.
My independent mechanic changed all the seals and gaskets that are part of the intake assembly including the EGR gaskets, fuel injector seals, throttle body, plenum, manifold gaskets, ect.. 23 gaskets and seals in all. It set me back $776.
Thanks for the responses Wingrider!